The four cities listed here are considered “megacities,” in which their population exceeds 10 million. Lahore, Pakistan is not listed in many of the lists of megacities. But I am sure it will be considered one in the near future. Air quality in Lahore isn’t pretty. Officially, Mexico City is the most polluted cities in the world. Pollutants way exceeding WHO standards are SO2 (Sulfur Oxides), Ozone and PM (Particulate Matter).